This invention relates to microwave filters and, more particularly, to a filter constructed as a cylindrical cavity with conically tapered end portions to provide a resulting resonator which is a cascade of two conical sections joined by a cylindrical section. The resulting filter provides increased bandwidth and reduced spurious response.
Microwave filters are employed widely in electromagnetic communication systems. For example, in satellite communication systems, the filters are used to define up-link and down-link communication channels. High Q microwave filters in the 3.7-4.2 GHz frequency range are currently constructed using TE.sub.111 cylindrical mode resonators. For certain applications, it is desirable to extend the passband down to 3.4 GHz.
A problem arises in that the presently available cylindrical resonator operating in the TE.sub.111 mode does not function adequately well over the entire band of 3.4-4.2 GHz band due to the presence of extraneous TM modes which resonate within the band. This results in a degradation of filter performance. As a result of this limitation, previous C-band work in the 3.4-4.2 GHz frequency range could be accomplished with a TE.sub.111 resonator only by dividing up the band into two sub-bands which might then be diplexed together, thereby to avoid the TM mode interference. However, such utilization of the resonator is not available in a communication situation requiring continuous use of the entire frequency band. Use of the entire frequency band requires that the resonator be free of a spurious mode over the entire band.